多孔SiO2凝胶玻璃的制备及孔径控制

以正硅酸乙酯（ＴＥＯＳ）为原料，采用溶胶－凝胶法制备了多孔ＳｉＯ２凝胶玻璃，研究了氨水，盐酸，硝酸，氢氟酸等不同种类的催化剂对孔的影响，制备了多孔ＳｉＯ２凝胶玻璃，利用氮气吸附法对孔结构进行了分析。在透射电镜下观察了凝胶颗粒形貌。     

Dependence of drying technique on surface area and pore size for polyethylene glycol/tetramethoxysilicate hybrid gels

The effect of drying technique on the surface area and pore size of ethylene glycol/polyethylene glycol (PEG)/tetramethoxysilicate hybrid gels is presented. Various molecular weight polyethylene glycols were added to acid-catalyzed solutions of tetramethoxysilane (TMOS) in methanol. The two drying techniques investigated were supercritical CO₂ drying and conventional evaporation. Nitrogen adsorption was used to determine the surface area and pore radius. As the molecular weight of the polyethylene glycol increased, the pore size of the gel increased, and the surface area decreased. For gels with no polymer additive, both the surface area and pore size of the gels is dependent on the drying method. However, when the various polymers were added, this difference became insignificant, indicating that the polymer assists in opening the pore structure and prevention of syneresis.     

Generalized free volume diagrams from positron annihilation data for the study of nanoporosity in silica gel specimens

Generalized positronium annihilation diagrams are proposed in this paper. In particular, we point out how the free volume plots arising from a very simple model can be utilized to determine the electronic layer thickness ΔR in which the ortho-positronium (o-Ps) annihilation events occur. We comment on the relevant role played by the Positronium Annihilation Lifetime Technique (PALT) as a powerful tool for studying diverse properties of materials and discuss the range of pore size distributions detectable by this technique. These considerations have been applied to the study of different silica gel specimens, prepared by the sol-gel technique. We found that the free volume curves fit well to the experimental data provided the thickness parameter ranges between ΔR_min= 0.71 nm and ΔR_max= 2.3 nm and also observed a bimodal pore size distribution which corresponds to the existence of two types of nanopores present in the silica gel samples. The variation of the mean free volume radius R with the chemical composition and the correlation between the positronium annihilation parameters and the potential catalysis properties of the modified silica gels, are also presented. Received: 23 June 2001 / Accepted: 23 July 2001     

Hydrophobicity of templated silica xerogels for molecular sieving applications

Silica xerogels were prepared by a sol-gel process catalyzed by acid with tetraethylorthosilicate, and using an organic covalent ligand template (methyltriethoxysilane) or a noncovalent template C6 surfactant (triethylhexylammonium bromide). The influence of hydrotreatment on the structure of templated xerogels is examined in terms of surface area, micropore volume, average pore size, and pore size distribution, and compared against a blank xerogel (nontemplated). The role of surface functional groups was evaluated using 29Si nuclear magnetic resonance. The structural integrity of the xerogel was maintained to a large extent in samples that had a high contribution of Q4 species (siloxane groups). Xerogel matrix densification occurred when there was a large concentration of Q3 and Q2 species (silanol groups), which also were responsible for increased hydrophilicity. The templated xerogels resulted in up to a 25% concentration of methyl functional groups (T3 and T2 species), leading to hydrophobic xerogels. The best results in terms of structural integrity and hydrophobicity were obtained with templated xerogels prepared with the C6 surfactant. The results in this study suggest that surfactant-enhanced condensation reactions lead to structures with a high contribution of Q4 groups, which are not susceptible to water attack, but are strong enough to oppose matrix densification during rehydration.     

Improvement in colloidal sol-gel SiO2 glass by attrition of aggregates in sols

The two-step colloidal sol-gel process for forming SiO₂ glasses has been improved by the use of attrition in ball mills consisting of borosilicate glass jars with fused SiO₂ cylinders as milling media. The average size of aggregates was reduced from more than 40 μm to less than 3 μm by milling up to 64 h depending on the hardness of the dried gel. Data on pore size distribution in the twice dispersed and dried gels show that 24 to 30 vol% of the pores are due to interaggregate spaces (pores whose sizes are larger than those represented by the average 16 nm pores between primary SiO₂ particles). These large pores cause a bimodal size distribution with the secondary peak at 3 to 4 μm with no milling and the position of the secondary peak decreases with milling until it is no longer resolved from the primary peak. The average size of the interaggregate pores is more than an order of magnitude smaller than the average size of the aggregates due to the efficiency of packing the aggregates of a broad size distribution. Milling is shown to markedly improve the transparency of the SiO₂ glass after sintering.     

Large- and small-nanopore silica prepared with a short-chain cationic fluorinated surfactant

A cationic partially fluorinated surfactant with four carbons in the chain 1-(3,3,4,4,4-pentafluorobutyl)pyridinium chloride is employed as a structure-directing agent to synthesize nanoporous silica. Samples are prepared in dilute ammonia solutions at room temperature with a range of surfactant:Si ratios. The sample with the largest surfactant:Si ratio forms particles with wormhole-like micropores with an average diameter of 1.6?nm, which corresponds to the anticipated small size of the surfactant aggregates. On the other hand, the sample with the smallest surfactant:Si ratio forms a gel that, upon drying, has uniform 11.1 nm pores. The formation and stabilization of the latter large-mesopore structure is unusual for a sample prepared and dried under ambient conditions, and may reflect favourable roles of the surfactant both in inducing gelation and in stabilizing the pore structure during drying.     

Templated titania films with meso- and macroporosities

Porous TiO₂ films with both mesoporosity and macroporosity were fabricated by a templated sol–gel method for applications, such as dye-sensitized solar cells (DSC), photocatalysis and catalysis. With the incorporation of differently sized pores, the resultant structures exhibit high surface areas and possess interpenetrating aligned pore channels, which are believed to be beneficial for applications where diffusion of reactants to interior surface can be rate limiting. Both liquid and solid TiO₂ precursors can be applied for large area coating in this process. Almost crack-free films were produced by templated coating of pre-synthesized TiO₂ nanoparticles. The measured specific surface area and porosity of synthesized films were in the range of 33–137 m²/g and 61–80%, respectively, depending on the size of the selected template.     

Thermal diffusion of radon in porous media

Based on the non-intersection model of cylindrical capillaries, the mean radius of the pores of some soils and building materials are estimated. In size, the above-mentioned radii are usually of the order of the free path of gas molecules at atmospheric pressure. A review of pore size distribution data also reveals that a large fraction of concrete pores belong to Knudsen's region. This fact indicates that the thermal gradient in these media must cause gas (radon) transport. The interpretation of the experimental data concerning the rate of emanation of 222Rn from a concrete-capped source subjected to a sudden increase in temperature is given, based on irreversible thermodynamics theory. The calculations given here for radon flux, caused by concentration and thermal gradients, are in satisfactory agreement with the experimental data. It is shown that thermodiffusion can significantly contribute to radon flux in concrete. The need to include the thermodiffusion radon flux in the radon entry model is discussed.     

Modeling interparticle and intraparticle (perfusive) electroosmotic flow in capillary electrochromatography

A model to estimate the extent of intraparticle, or perfusive, electroosmotic flow (EOF) in CEC capillaries packed with macroporous particles has been developed. Nucleosil packings (d(p) = 7 microm) having nominal pore sizes of 500, 1000, and 4000 A were studied. Intraparticle pores ranging from 50 to 10000 A in diameter were partitioned into 995 intervals of 10 A. Using pore size distribution data for the sorbents obtained by mercury intrusion porosimetry, fractions of the total column void volume contributed by pores in the range of interest were determined. The average channel diameter of the interstitial space was estimated from the d(p) of the packing; its fraction of total column volume was determined from the interstitial porosity. Estimations of relative EOF velocity in the intraparticle and interstitial channels were made by treating the channels as parallel cylindrical capillary tubes. Relative EOF values were combined with the volume fraction data and used as weighting factors in calculating an effective particle diameter (d(p,eff)) for each set of conditions (i.e., packing type, ionic strength of eluent). Values of d(p,eff) generated by the model correctly predict the trends observed in the experimental data. At the lowest ionic strength, plate height correlated inversely with the pore size of packing (h4000 A < h1000 A < h500 A). Rate curves for each column tended toward lower plate heights with increasing eluent ionic strength before converging at some limiting point. The point of convergence was reached at moderate ionic strengths for the larger pore media (1000 and 4000 A) and higher ionic strength for the 500 A.     

Design of Porous Alginate Hydrogels by Sacrificial CaCO3 Templates: Pore Formation Mechanism

Fabrication of porous alginate hydrogels with a well‐controlled architecture useful for tissue engineering is still a challenge. Here, CaCO₃‐based templating is utilized to design stable alginate gels with controlled pore dimensions in the range of 5–50 μm. The mechanism of pore formation is studied considering two factors affecting the pore size: i) osmotic pressure generated during the dissolution of sacrificial CaCO₃ templates and ii) alginate gel network density. Osmotic pressure can achieve an upper limit of 100 MPa but does not affect the gel porosity. Additional osmotic pressure (range of kPa) induced by dextrans pre‐encapsulated into CaCO₃ vaterite is also insufficient for pore enlargement. Pore stability depends merely on the gel network density and on the number of crosslinking calcium ions provided locally per unit time; pores are collapsed when template dissolution is too slow or if there is insufficient alginate concentration (below 2%). Young's modulus indicates the soft nature of the prepared hydrogels (tens of kPa) applicable as soft porous scaffolds with a tuned internal structure.     

Blending effect of sucrose and surfactant templating agents on silica mesostructure

We report the blending effect of surfactant and sucrose as a nonsurfactant templating agent on the silica mesostructure. The CTAB/sucrose-templated mesoporous silica (SCS) was compared with CTAB-templated MCM-41. The MCM-41 showed spherical morphology with a particle diameter of 1.1–1.5 μm, and gave a bimodal size distribution, centered at 2.1 nm and 8.9 nm, which is assigned to hexagonally-arrayed cylindrical pores and interparticle-pores between small MCM-41 clusters, respectively. SCS gave unique and extraordinary morphology in which two different mesostructures have grown with both of them facing each other. The ordered MCM-41 pore structure clung to silica nanosphere-framed wormlike mesostructure, resulting in a bimodal pore size distribution centered at 2.1 nm and 7.0 nm. It was revealed that both of CTAB and sucrose act independently as a surfactant and a nonsurfactant template.     

A model for the porosity dependence of Young's modulus in brittle solids based on crack opening displacement

A crack opening displacement concept has been introduced to model the porosity dependence of Young's modulus in polycrystalline and single phase solids. In developing the theoretical model, it is assumed that each cylindrical cavity possesses radial cracks and spherical pores possess annular flaws. When an external stress is applied on such a solid, its elastic response is shown to be governed by the pore size, the width of an annular flaw, the number of pores (or pore volume fraction) and the flaw to pore size ratio. The validity of the present approach is tested against a number of experimental data.[/p]     

The importance of dehydration in determining ion transport in narrow pores

The transport of hydrated ions through narrow pores is important for a number of processes such as the desalination and filtration of water and the conductance of ions through biological channels. Here, molecular dynamics simulations are used to systematically examine the transport of anionic drinking water contaminants (fluoride, chloride, nitrate, and nitrite) through pores ranging in effective radius from 2.8 to 6.5 Å to elucidate the role of hydration in excluding these species during nanofiltration. Bulk hydration properties (hydrated size and coordination number) are determined for comparison with the situations inside the pores. Free energy profiles for ion transport through the pores show energy barriers depend on pore size, ion type, and membrane surface charge and that the selectivity sequence can change depending on the pore size. Ion coordination numbers along the trajectory showed that partial dehydration of the transported ion is the main contribution to the energy barriers. Ion transport is greatly hindered when the effective pore radius is smaller than the hydrated radius, as the ion has to lose some associated water molecules to enter the pore. Small energy barriers are still observed when pore sizes are larger than the hydrated radius due to re‐orientation of the hydration shell or the loss of more distant water. These results demonstrate the importance of ion dehydration in transport through narrow pores, which increases the current level of mechanistic understanding of membrane‐based desalination and transport in biological channels.     

Pore water distribution in mortar during drying as determined by NMR

The drying of a saturated mortar sample was measured using Nuclear Magnetic Resonance. NMR is a technique which gives the total moisture content of a specific volume at a certain time. By looking also at the relaxation of behaviour of the NMR signal, more information can be obtained. For every time and every position, the pore size distribution of the mortar was measured. Other techniques show that NMR is a reliable method to measure pore size distributions. Two distinctive pore sizes can be seen in the pore size distribution. This corresponds to the microstructure of mortar, which has small (< 10nm) gel pores and bigger) ≈ 10 to 1000 nm) capillary pores. A one-dimensional drying experiment was carried out by blowing dry air (relative humidity =0+5%; temperature=20±1°C) ove the top of a cylindrical mortar sample, while all the other surfaces were sealed to prevent drying in other directions. The changing moisture content in the pores was followed with time during drying for about 3 days. This leads to the conclusion that the water cannot be extracted from the gel pores, while the capillary pores dry within about 20 hours under the drying conditions and time adopted.     

Concentration and temperature effect on controlling pore size and surface area of mesoporous titania by using template of F-68 and F-127 co-polymer in the sol-gel process

Mesoporous titania with crystalline pore walls and controlled pore sizes was fabricated through triblock copolymer (pluronic series) templated sol-gel process by changing the copolymer concentration and by adjusting their calcination temperature. Compared with mesoprous silicate, the synthetic condition of mesoporous titania would be sensitive to calcination temperature. Their pore arrangement and pore size depend strongly on the concentration of copolymer used as a template. Their arrangement of pores and specific surface area increases with the increase of calcination temperature up to critical limit, 320 degrees C. Beyond the critical temperature, the orderness of pores and specific surface area decreases due to the collapse of the pore walls. The specific surface area, pore size and pore orderness can be controlled by optimizing calcination temperature as well as polymer concentration. We demonstrate the mechanism of pore formation and their collapse in the sol-gel synthesis of mesoporous titania.     

Transformation of the geopolymer gels to crystalline bonds in cold-setting refractory concretes: Pore evolution,mechanical strength and microstructure

Two K₂O–MgO–Al₂O₃–SiO₂ based geopolymer gels with bulk chemical composition corresponding to cordierite (Co) and 1:1 mullite-cordierite (MuCo) were successfully transformed to crystalline bonds in high temperature service of cold-setting made refractory concretes. Kyanite aggregates changed the flexural strength of the gels from 11 to 28 MPa due to the development of good adhesive bonds. Under thermal cycles, up to 1250 °C, the cumulative pore volume remained at 0.09 mL/g, as from the absence of important densification/shrinkage. However, the behavior of the cumulative pore volume curves changed from that of a matrix with a wide range of distribution of pore sizes to that, of matrix, consisting of relatively coarse grains. The latter exhibits a rise at 10 μm as void spaces created around the contact points among the coarse kyanite grains and that at 0.054 μm as pores within the crystalline phases (cordierite, kalsilite, leucite, mullite, enstatite) formed. The microstructural observations confirmed the transformation of gel pores (size around 0.01 μm) to interparticle and intergranular pores due to the crystallization. The flexural strength of refractory concretes increased from 28 MPa to 40 MPa in agreement with the increase in the elastic modulus from 9 to 30 GPa. The crystallization was enhanced by the MgO content (being important in Co compared to MuCo) and the kyanite concentration as particles of kyanite effectively acted as phase separation and nucleation sites.     

Processing effect on structural changes of high acid-catalysed silica gel

The structure of silica gels prepared by hydrolysis of tetraethoxysilane (TEOS) with high acid content has been investigated. The gels were obtained by varying the nitric acid contents at a constant H₂O/TEOS molar ratio of 10. The molar compositions were TEOS: H₂OHNO₃ =110: x(x=0.1–1.0). The effects of acid content and drying temperature on structural changes in the gels were examined. In addition, the surface area, pore volume, pore size distribution, and the microstructure were determined. The results showed that the pore structure changed to larger pore sizes and broader size distributions as a result of increasing the drying temperature. However, at a constant drying temperature, a higher acid content yielded a higher pore volume. The gels dried at lower temperatures had slit-shaped micropores and a narrow pore size distribution; whereas, the gels dried at higher temperatures had ink bottle-shaped pores and a relatively broad size distribution.     

SAXS analysis of the effect of H2 dilution on microstructural changes of HWCVD deposited a-SiC : H

Hydrogenated amorphous silicon carbon (a-SiC : H) films were deposited by hot wire chemical vapor deposition (HWCVD) and their porosity was investigated by small-angle X-ray scattering (SAXS), XRR and FTIR. SAXS measurement was analyzed by Guiner plot, Porod plot, Debye plot and scaling factor. The measurement of the SAXS results assumed a distribution of spherical pores. This analysis suggested that the maximum of pore size distributions occur for radius of gyration of 5–6 Å. As the H₂ flow rate increases, the pore size distribution narrows and the volume occupied by the pores decreases. A direct relation between the atomic density of a-SiC : H films deposited by HWCVD and the pore volume fraction was also obtained. The low scattering intensity observed for the films deposited by HWCVD showed that they were compact and homogeneous regardless of the H₂ dilution.     

Properties of hydrothermally modified silica gels — Effect of the parameters of porous structure on the course of the modification process

The effect of the parameters of globular structure (diameter of globules and number of their contact points) on the course of a texture modification process has been examined. Modification of silica gels was carried out in an autoclave at elevated pressure in the temperature range 110–300°C and in a water vapour atmosphere. Silica gels having specific surface area of 64–830 m²/g and predominant pore radii of 40–1000 Å were modified. The mechanism of hydrothermal treatment is relatively complex. It depends, among other things on different parameters of globular texture, the temperature of a process and on the presence of different admixtures in the adsorbent being modified. Above 200°C the course of hydrothermal modification depends only slightly on the nature of the silica gel (narrow or wide porous).     

Ti-C含量对多孔TiC/FeAl复合材料孔型结构和力学性能的影响

以尿素为造孔剂,利用自蔓延高温合成技术制备了多孔TiC/FeAl复合材料,主要考察了Ti-C含量（质量分数为15wt%~35wt%）对多孔TiC/FeAl复合材料孔型结构和压缩性能的影响。当Ti-C含量不高于25wt%时,多孔TiC/FeAl复合材料由毫米孔和孔壁微孔组成规则的复合孔型结构。相互连通的毫米孔产生于尿素颗粒的挥发和液相迁移;微孔尺寸为10~50 μm,产生于Fe-Al-Ti-C粉末的自蔓延过程,孔径随Ti-C含量的增加而增大。通过调整尿素的体积分数,多孔TiC/FeAl复合材料的孔隙率可控制在56.64%~85.35%。当Ti-C含量不高于25wt%时,多孔TiC/FeAl复合材料的抗压强度随Ti-C含量的增加而增大。当Ti-C含量高于25wt%时,多孔TiC/FeAl复合材料壁面微孔形状很不规则,且抗压强度下降。孔隙率约为64.3%时,多孔Fe-Al金属间化合物和TiC/FeAl复合材料（Ti-C含量为25wt%）的抗压强度分别为20.03 MPa和66.68 MPa,对应的应变值分别为4.77%和8.21%。另外,多孔TiC/FeAl复合材料的压缩性能可用Gibson-Ashby模型来解释。